The invention relates to a system for monitoring a subsea oil or gas reservoir by means of measuring instruments located in a completed well.
In exploring for oil or gas, particularly at great depths, it is desirable to know as much as possible about the hydrocarbon-bearing formation. In the initial phase of exploring for oil and gas this is done by means of seismic surveys, which provide an “image” of the formation. By analysing these images, geologists can then see whether there are oil and/or gas-bearing formations under the ground. However, it will not be possible to obtain a certain indication until wells have been drilled down to the reservoir.
During drilling further information can be obtained about the formation (MWD=Measurement While Drilling). Measurements are carried out, for example, of pressure, temperature, resistivity and gamma waves. Another method normally employed for obtaining information about the reservoir is to take core samples of the rock at regular intervals. If oil is discovered, samples thereof will also be taken. These samples provide valuable information concerning the nature of the reservoir, its pore size (which indicates the utilisation factor), its permeability, how the oil is distributed in the reservoir, etc., all in order to obtain information about the quality of the oil and to some extent also the size (extent) of the reservoir together with the prospects for utilisation.